This application is based on application No. 2000-11344 filed in the Korean Industrial Property Office on Mar. 7, 2000, the content of which is incorporated hereinto by reference.
(a) Field of the Invention
The present invention relates to a negative active material for a rechargeable lithium battery and a method of preparing the same and, more particularly, to a negative active material and a method of preparing the same in which the negative active material exhibits good electrochemical performance.
(b) Description of the Related Art
Metallic lithium was initially used as the negative active material in rechargeable lithium batteries. However, metallic lithium causes an abrupt loss in capacity and deposits to form a dendric layer which induces a short circuit between the negative and positive active materials and a reduction in the cycle life of the battery. There have been attempts to use lithium alloy instead of metallic lithium. However, problems encountered with the use metallic lithium nevertheless remain and are not substantially minimized.
Recently, carbonaceous material into or from which lithium ions are intercalated or deintercalated is used as the negative active material. There are two main types of carbon-based materials used crystalline graphite and amorphous carbon. Crystalline graphite includes artificial graphite and natural graphite. Typical examples of artificial graphite include mesocarbon fiber (MCF) and mesocarbonmicro beads (MCMB). There has been introduced somewhat recently artificial graphite, which is doped with a different element from carbon to produce an inexpensive negative active material exhibiting good electrochemical performance.
Mesophasecarbon microbeads and mesophase fiber are easily produced but are expensive. Also, because of a stabilizing step that must be performed, mesophasecarbon microbeads and mesophase fiber have substantially lower discharge capacities than natural graphite. Natural graphite on the other hand, has a high discharge capacity, but a higher irreversible capacity than mesophasecarbon microbeads or mesophase fiber. Also, because natural graphite is plate-shaped, discharge properties are extremely deteriorated.
To overcome such problems, various studies have been conducted on shapeless and low coke-based artificial graphite in which elements such as boron are used as a doping material to enhance the overall performance of the battery.
Japanese Patent Laid-open No. Hei 3-165453 (Central Devices Co.) discloses a method in which a BCN(H) type negative active material is produced using a CVD (Chemical Vapor Deposition) procedure.
Japanese Patent Laid-open No. Hei 3-245458 (Sony) discloses a method in which organic materials are carbonized by adding 0.15 to 2.5wt % of B compounds using an autoclave procedure to prepare a negative active material.
Japanese Patent Laid-open No. Hei 5-266880 (Matsushita) discloses a method in which B, B2O3 or H3BO3 is added to petroleum pitch and the mixture is heat-treated at 2000xc2x0 C. for 20 hours to prepare a negative active material.
Japanese Patent Laid-open No. Hei 9-63584 (Petoca) and Hei 9-63585 (Petoca) disclose methods in which, after pulverizing mesophase fiber, B compounds are added to the fiber and the mixture is heat-treated.
However, in all of the above methods, a negative active material results that is deficient in at least one important aspect including capacity, charge and discharge efficiency, current density, and cycle life.
It is an object of the present invention to provide a negative active material for a rechargeable lithium battery, which exhibits good high-rate characteristics and cycle life characteristics.
It is another object to provide a method of preparing the negative active material for a rechargeable lithium battery.
These and other objects may be achieved by a negative active material for a rechargeable lithium battery including a crystalline carbon core and a semi-crystalline carbon shell formed surrounding the core. The semi-crystalline carbon shell includes at least one element and carbides thereof or a compound of at least two different types of elements, and has a turbostratic or half onion-sheath structure. The element (hereinafter referred to as a catalyst element) serves graphitization catalyst and causes a change in a structure of surrounding carbon. The compound includes only catalyst elements selected from transition metals, alkaline metals, alkaline earth metals, semi-metals of Group 3A of the Periodic table, semi-metals of Group 3B, semi-metals of Group 4A, semi-metals of Group 4B, elements of Group 5A, elements of Group 5A, lanthanides or actiniums.
The negative active material may include metal boride and metal carbide.
In order to achieve these objects and others, the present invention provides a method of preparing a negative active material for a rechargeable lithium battery. In this method, amorphous carbon or crystalline carbon is mixed with at least one catalyst element and carbides thereof, or compound of at least two different types of elements. The compound of the elements includes an A-B compound and an A-C compound. A and B are respectively at least one catalyst element and C is not a catalyst element. Thus, the A-B compound refers to a compound including only catalyst elements and the A-C compound refers to a compound including another component in addition to the catalyst elements. The catalyst element acts as a graphitization catalyst and causes a change in a structure of surrounding carbon. Following the mixing step, the mixture is graphitized at 2000 to 3100xc2x0 C.